1. Field of the Invention
The present invention relates to an ink composition, particularly to an ink composition that results in a good glossiness on glossy media and solves the problem of uneven glossiness by reducing the differences in glossiness between the colors, and an ink set having such ink composition.
2. Description of Prior Art
An inkjet recording method is a method of printing by jetting drops of an ink composition onto a recording medium such as paper. A feature of this method is that high-resolution and high-quality images can be printed at high speed using a relatively inexpensive apparatus. Ink compositions commonly used in the inkjet recording method typically contain water as their main ingredient, mixed together with coloring components and wetting agents such as glycerin to prevent clogging. Dye inks and pigmented inks can be used as colorants. However for printed matter, since pigmented inks are superior to dye inks in terms of weathering resistance such as light fastness, water fastness and gas fastness, the demand for pigmented inks is increasing. Therefore, the development of pigmented inks in which the characteristics of pigments are utilized, has been in progress.
With recent innovative improvements in inkjet recording technology, the inkjet recording method has been applied in the field of high-definition printing that had already been achieved with the silver halide photo printing method and the offset printing method. Accordingly, developments have also been made in the area of inkjet recording media where high glossiness comparable to photographic paper and art paper which are used in the silver halide photo printing method and the offset printing method, has been achieved. As for high-glossy recording media for inkjet printing, the mainstream is a substrate (e.g., paper, film, etc.) having thereon an ink-absorbing layer that contains porous material such as porous silica.
When forming color images using the inkjet recording method, ink compositions of three colors, yellow, magenta, and cyan, are used, or in some cases, four colors including a black ink composition. There are also cases where a light cyan ink composition and a light magenta ink composition are added to the four ink compositions listed above, so that color images are formed by using six colors. When printing to a recording medium using a plurality of color ink compositions, if the respective ink compositions have widely differing ink penetrations (i.e., ink absorption speeds) into the recording medium, the printed images usually suffer from bleeding, uneven colors, uneven flocculation, and uneven glossiness, thus the quality of the recorded images is reduced.
Compared to dye inks, pigmented inks in which pigments serve as colorants are more prone to uneven flocculation, since the pigments are layered on the surface of a recording medium.
According to the definition in Patent Laid-Open Publication No. 2002-138232, color unevenness refers to the unevenness in the image quality when overlapping colors as secondary and tertiary colors due to the ink components having different absorption speeds, respectively. Uneven flocculation refers to image ruoughness (graininess) when dots of different colors overlap each other due to the respective ink components having different absorption speeds. Uneven glossiness refers to the differences in glossiness due to the differences in absorption speeds between the respective ink components.
As a solution for these problems, the same patent teaches that, in an ink set having multiple pigmented ink compositions, the absorption coefficients of the respective ink compositions are limited within a predetermined range, so that uneven colors, uneven flocculation, and uneven glossiness are reduced. The absorption coefficients are indicated in the testing method approved by the Japan Technical Association of the Pulp and Paper Industry, that is, “Bristow's method” (JAPAN TAPPI No.51) (for example, see page 350 of Shin Kami no Kagaku (New Science of Paper) 4th edition, co-written by Takashi Kadoya et al.).
The same patent discloses an ink composition containing, as a penetrating agent, an acetylene glycol compound and a polyhydric alcohol alkyl ether derivative having three or more carbons.
A number of pigmented ink compositions using pigments as colorants have been proposed, one of which is the ink composition disclosed in Patent Laid-Open Publication No. 2001-354889, that exhibits adequate penetration and ink discharge stability by containing glycol ether and 1,2-alkandiol.
Patent Laid-Open Publication No. H11-349871 discloses an ink composition containing a colorant, an aqueous carrier, an alkanediol having hydroxy groups at the each terminal, a polyethylene glycol with a molecular weight of about 200 to 5000, a mixture of polyethylene glycol and polypropylene glycol, and a polyol/polyalkylene oxide condensate having a specified structure. These patents show that the respective ink compositions provide an excellent optical density for printed matter and good printing properties for inkjet printers.
However, these ink compositions still have room for improvement in relation to the glossiness of images when printed on glossy media having ink-absorbing layers. Therefore, it is desired that an ink composition that exhibits excellent glossiness on glossy inkjet media should be developed.